I'm so glad this hidden monorail broke down.

A video on YouTube. In Explainers, a Krater category.

Summary by Krater

Tom Scott takes a maintenance tour inside the hollow interior structure of the Prince of Wales Bridge, exploring the hidden monorail system known as the rat.

From the video

Answers: What is inside the Prince of Wales Bridge?

Prince of Wales Bridge
Severn estuary
Cable-stayed bridge
Bridge maintenance
Monorail rat system

What it concludes

The Prince of Wales Bridge features a hidden suspended monorail system called the rat used for maintenance inspection.
The bridge interior contains post-tensioning cables that are kept under permanent compression to keep the structure strong.

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